class MayBe {
    static of (value) {
        return new MayBe(value)
    }
    constructor (value) {
        this._value = value
    }
    map (fn) {
        return this.isNothing() ? MayBe.of(null) : MayBe.of(fn(this._value))
    }
    isNothing () {
        return this._value === null || this._value === undefined
    }
}
let r = MayBe.of(null)
          .map(x => x.toUpperCase())
console.log(r)